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#1 danyal123

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Posted 15 September 2021 - 08:20 AM

Hello everyone,

                        I've been observing for almost 6 months now, mostly looking at planets and star clusters, and I have a question related to seeing and its effects. So I use several seeing forecasts to predict my seeing for the night, and I wanted to ask, what is the best possible seeing that my telescope (6 inch) can benefit from? I was reading a bit and I think the resolution limit of my telescope is around 0.7 arcseconds, so whenever my seeing forecasts show that seeing is 0.7 arcseconds, can I be fairly certain that my telescope will be performing at its limit on such nights? 

Please guide me on this matter.

 

Regards,

Danyal


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#2 Mike G.

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Posted 15 September 2021 - 08:41 AM

my experience is that seeing forecasts are typically like local weather forecasts.  except less accurate.  with seeing, you have lots of very local conditions like heat off of rooftops and parking lots to reduce your viewing conditions.  also, even with a 6" reflector, you need to acclimate the scope or you will not have a good image.  Seeing varies minute by minute, even second by second.  the Starblast is a decent scope and you have a good selection of EP's for it.  make sure it's collimated well and thermally acclimated then just use the EP that produces the best image for the target you have.  I adjust for seeing conditions by changing magnification.  Clouds are a bit more difficult to deal with.


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#3 Jethro7

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Posted 15 September 2021 - 09:01 AM

Hello Danyal,

There are a lot of things going on that effect your  views. You have been looking up at the night sky for a bit now. Is the real issue " I want to see more"?

 

HAPPY SKIES AND KEEP LOOKING UP Jethro


Edited by Jethro7, 15 September 2021 - 09:02 AM.


#4 Keith Rivich

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Posted 15 September 2021 - 11:37 AM

Hello everyone,

                        I've been observing for almost 6 months now, mostly looking at planets and star clusters, and I have a question related to seeing and its effects. So I use several seeing forecasts to predict my seeing for the night, and I wanted to ask, what is the best possible seeing that my telescope (6 inch) can benefit from? I was reading a bit and I think the resolution limit of my telescope is around 0.7 arcseconds, so whenever my seeing forecasts show that seeing is 0.7 arcseconds, can I be fairly certain that my telescope will be performing at its limit on such nights? 

Please guide me on this matter.

 

Regards,

Danyal

From my experience, depending on what magnification you tend to use, 2" seeing and better would be good for a 6" scope. 


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#5 sevenofnine

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Posted 15 September 2021 - 11:43 AM

I only use Clear Sky as a general forecast. Like the weatherman saying "cloudy today with a chance of showers." Helpful but not too accurate. I evaluate the sky conditions overhead using this method. If bright stars look like planets with no twinkling then atmospheric turbulence is low. If I can see Megrez in the Big Dipper clearly then that's another good sign. These two tests help no matter what scope you use. Best of luck to you! waytogo.gif


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#6 Dave Mitsky

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Posted 15 September 2021 - 12:10 PM

In case you haven't seen them yet, danyal123, here are some links to articles on seeing.

http://www.damianpeach.com/seeing1.htm

 

http://www.damianpea...m/pickering.htm

 

https://skyandtelesc...ing-the-seeing/

 

https://www.skyatnig...nomical-seeing/


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#7 Sketcher

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Posted 15 September 2021 - 12:43 PM

the resolution limit of my telescope is around 0.7 arcseconds, so whenever my seeing forecasts show that seeing is 0.7 arcseconds, can I be fairly certain that my telescope will be performing at its limit on such nights? 

Absolutely not!

 

Even if seeing forecasts were 100% accurate (which they are far far from) you would not be able to be confident of your telescope performing at its best -- not even on a night of perfect seeing conditions.  There are too many other factors that can (and often will -- especially for less experienced telescope users) compromise your telescope's performance.

 

I've been using telescopes for more than 50 years; and I've never relied on seeing forecasts.  I don't even look at them.  They're that unreliable!  Instead, I rely on experience.  If I want to go out and observe, I go out and observe.  I take my chances with the seeing conditions.  And, strangely enough, with enough experience one's eye-brain system actually learns how to filter out the good from the bad even under seeing conditions that are so poor than one would have scrapped the observing session entirely when one was less experienced.

 

That being said, the two, perhaps most commonly used, seeing scales are the Pickering (or Modified Pickering) Scale and the Antoniadi Scale.  Look them up!  Practice using them, and eventually you'll be able to quite accurately judge the real-time seeing conditions while you're at your telescope.

 

But then there are other phenomena that can more or less mimic the effects of astronomical seeing -- at least until one learns how to tell them apart (experience and knowledge come into play -- once again).

 

There's really no end to learning when it comes to this hobby.  This can be a source of frustration; but it can also serve to maintain one's interest in the long term.  Some people enjoy learning new things.  Some just want to see stuff -- and see it now.  Some stick with the hobby for a lifetime.  Some give up early on.

 

Knowing what's going on with one's telescope in real time, when it's being used, is one of the toughest things to learn when starting out:  "Is the seeing bad, or does my telescope have poor optics, or is my telescope poorly collimated, or are my optics not sufficiently cooled down, or is my telescope suffering from tube currents, etc."

 

When first starting out, this is a complex, convoluted mess that's impossible to unravel; but with patience, with time, with experience, and with the right resources (such as a thorough star-testing guide) this mess will gradually sort itself out and you'll know what's going on.  But there's no "instant" solution to this.  It's going to take time.


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#8 aeajr

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Posted 15 September 2021 - 03:27 PM

Hello everyone,

                        I've been observing for almost 6 months now, mostly looking at planets and star clusters, and I have a question related to seeing and its effects. So I use several seeing forecasts to predict my seeing for the night, and I wanted to ask, what is the best possible seeing that my telescope (6 inch) can benefit from? I was reading a bit and I think the resolution limit of my telescope is around 0.7 arcseconds, so whenever my seeing forecasts show that seeing is 0.7 arcseconds, can I be fairly certain that my telescope will be performing at its limit on such nights? 

Please guide me on this matter.

 

Regards,

Danyal

As others have said, there are so many factors that can affect what you see.  Seeing is only one of them.   Transparency is very important.  Sky Glow and light pollution can be major factors.  Dew can be a big issue if you can't keep your eyepieces and scope from fogging up.

 

Nothing wrong with asking about seeing, just don't get overly focused on it.

 

The higher the mag you are using the more seeing will impact the image.  Sometimes dropping down a mag level is the best choice to get the best image.   I like to use a zoom eyepiece so I can vary the magnification at will. 


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#9 danyal123

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Posted 17 September 2021 - 12:38 AM

Thanks everyone for the replies. I agree that there are a lot of factors that contribute to the astronomical image, and also that seeing forecasts are probably not that accurate.

In hindsight I think that my question was not phrased correctly, what I meant to ask is hypothetically what is the best seeing that my telescope can benefit from? Like what is the level of seeing after which my telescope becomes effectively diffraction limited?

I've had a couple of nights when various seeing forecasts showed 1 arcsecond seeing, and on these nights I could see the airy disk along with a complete first ring, but when I was observing Jupiter I still got the impression that I was missing something. Jupiter showed good detail in such conditions, but sometimes the image would waver slightly and Jupiter would show even more detail. So is 0.7 arcsecond seeing good enough(which is the resolution limit of my scope),  or do I need better seeing like 0.35 arcseconds so that my telescope is effectively diffraction limited?



#10 aeajr

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Posted 17 September 2021 - 07:03 AM

Thanks everyone for the replies. I agree that there are a lot of factors that contribute to the astronomical image, and also that seeing forecasts are probably not that accurate.

In hindsight I think that my question was not phrased correctly, what I meant to ask is hypothetically what is the best seeing that my telescope can benefit from? Like what is the level of seeing after which my telescope becomes effectively diffraction limited?

I've had a couple of nights when various seeing forecasts showed 1 arcsecond seeing, and on these nights I could see the airy disk along with a complete first ring, but when I was observing Jupiter I still got the impression that I was missing something. Jupiter showed good detail in such conditions, but sometimes the image would waver slightly and Jupiter would show even more detail. So is 0.7 arcsecond seeing good enough(which is the resolution limit of my scope),  or do I need better seeing like 0.35 arcseconds so that my telescope is effectively diffraction limited?

Interesting.

 

I have never paid attention to seeing forecasts in arcseconds because it varies too much over time and location.   A 1-5 scale or above and below average forecast seems to be as good as it gets for me, here on Long Island, outside NYC. 

 

I have watched seeing improve or degrade over a couple of hours.  I have had reports of excellent seeing turn up disappointing results and poor seeing forecasts coincide with a pretty good evening of observing.

 

Such forecasts are averages over huge expanses of sky.  But the little part of the sky that my scope is seeing may vary quite a bit from that big average.

 

If seeing forecast is poor, I shift my plans from planets to clusters and less challenging doubles.  But that doesn't keep me from checking out the planets anyway, just in case my local conditions are much better than the forecast.


Edited by aeajr, 17 September 2021 - 07:04 AM.


#11 Tony Flanders

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Posted 17 September 2021 - 07:07 AM

I'm not sure I've ever been in a situation where a 6-inch scope would actually resolve to its full potential. The better the seeing, the more you will see.
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#12 Asbytec

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Posted 17 September 2021 - 07:27 AM

I'm not sure I've ever been in a situation where a 6-inch scope would actually resolve to its full potential. The better the seeing, the more you will see.

Absolutely. I stumbled into an area of great seeing after retirement, an area where Pickering 8/10 (as seen in a 6") was common several times a week. Better seeing was not once in a life time, more like weekly. I felt blessed, actually, despite my own folly of not knowing what I was getting into. Seeing so good, you could watch the Poisson spot defocus into a small diffraction ring and re-form at the center of that ring. Once I saw the first diffraction ring just a wave or so from focus settling into - not one bright ring - but, just for a moment, that one bright ring appeared to be composed of several thin rings. That lucky snapshot stunned me. Once I tried to see if the diffraction ring of a companion would overlap the dark space of the primary double star. It doesn't, it just forms a figure 8. But, seeing was good enough to see that clearly. Craters smaller than Dawes (for a 6" aperture), detail in the Martian North Polar Cap including a (large) fissure, and albedo features on Ganymede. Jupiter rocked on the Zenith. Saturn offered no Enke gap, but Enceladus flickered into view twice during an apparition years ago. Just amazing stuff to see when seeing is very good in the aperture we have. In near lab-like conditions you really get to see what your scope can do, and it's not disappointing despite the modest aperture, any obstruction, or with some aberration present.


Edited by Asbytec, 17 September 2021 - 07:30 AM.

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#13 Starman1

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Posted 18 September 2021 - 04:11 PM

Hello everyone,

                        I've been observing for almost 6 months now, mostly looking at planets and star clusters, and I have a question related to seeing and its effects. So I use several seeing forecasts to predict my seeing for the night, and I wanted to ask, what is the best possible seeing that my telescope (6 inch) can benefit from? I was reading a bit and I think the resolution limit of my telescope is around 0.7 arcseconds, so whenever my seeing forecasts show that seeing is 0.7 arcseconds, can I be fairly certain that my telescope will be performing at its limit on such nights? 

Please guide me on this matter.

 

Regards,

Danyal

For double stars, 4.5/6 = 0.75" is the Dawes limit on resolution.

For resolution of small details with high contrast (like Cassini's Division in the rings of Saturn), it can be smaller than that.

For typical viewing, 1-2" is about average.

 

One thing to remember is that seeing fluctuates.  On a given night, you could have some 0.5" moments and some 2.5" moments, with an average about 1.2".

When seeing is specified, it usually is referring to the lowest common denominator, as seen in long images.  A visual observer will catch those fleeting moments of

great seeing, while the camera will not.

Seeing is best at most sites between midnight and dawn, when the atmosphere quits down.

It is often best in the center of a broad valley, not the mountains on either side.

It can be quite good in the center of a large city if the warm air dome has settled down.

It is often best 2-3 days after a front has come through and the air motion seems calm and the air stagnant.

 

Learn to read weather maps to predict seeing, written by a weatherman astronomer:

http://www.cloudynig...observing-r1396
http://www.cloudynig...udy-night-r1413

http://www.cloudynig...observing-r1436


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#14 spkerer

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Posted 18 September 2021 - 08:19 PM

Thanks everyone for the replies. I agree that there are a lot of factors that contribute to the astronomical image, and also that seeing forecasts are probably not that accurate.

In hindsight I think that my question was not phrased correctly, what I meant to ask is hypothetically what is the best seeing that my telescope can benefit from? Like what is the level of seeing after which my telescope becomes effectively diffraction limited?

I've had a couple of nights when various seeing forecasts showed 1 arcsecond seeing, and on these nights I could see the airy disk along with a complete first ring, but when I was observing Jupiter I still got the impression that I was missing something. Jupiter showed good detail in such conditions, but sometimes the image would waver slightly and Jupiter would show even more detail. So is 0.7 arcsecond seeing good enough(which is the resolution limit of my scope),  or do I need better seeing like 0.35 arcseconds so that my telescope is effectively diffraction limited?

Jupiter is somewhat low in the sky hitting max of 36 degrees above the horizon currently for me.  Seeing can vary across the sky, but you can pretty much count on seeing being worse the closer to the horizon you're viewing.




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